Taper thread splices or joints are widely employed on concrete reinforcing bar used in concrete construction. One well known such bar splice is sold under the trademark LENTON by Erico Products, Inc. of Solon, Ohio or Erico BV of Tilburg, Netherlands. The splice utilizes a coupler or sleeve having internal taper threads mating with precision taper threaded ends of the bars to form full tension butt splices. Such splices are available for all sizes of reinforcing bar from #3 (10 mm) to #18 (57 mm).
The taper threaded bar ends may be prepared on a bar threading machine such as shown in U.S. Pat. No. 4,526,496 and also sold under the trademark LENTON. Similar machines may be seen in prior U.S. Pat. Nos. 4,282,618 and 4,213,722.
The LENTON bar threaded utilized a rotary head with a square opening accommodating four interfitting chaser holders. The head is indexed axially of a fixed bar end. The chaser holders are moved in the head by an axially extending control key or linear cam through a spring held cam block mounted for sliding movement on a transverse rod. The control key includes projecting rollers which engage a ring.
While the machine described above satisfactorily forms precision tapered threads on bar ends, it may require several passes to form the threads on large bar ends. It also requires substantial and fairly complex changes or set-up to switch from one bar size to another. For example, up to four passes may be required for some bar sizes and set-up for a different bar size may require twenty to thirty minutes, and a skilled operator. On some bar sizes the chaser holders, cam blocks and springs have to be changed, and for each bar size the rather inaccessible control key requires changing. Moreover the bar ends usually have to be saw cut or otherwise prepared, rather than simply sheared.
The excessive work done by the chasers results in excessive wear and more frequent replacement, which again results in downtime or production inefficiency.
It would therefore be desirable to have a machine which would thread a bar end in one pass within an optimum cycle time and in which the set-up required for converting the machine from one bar size to another could be shortened and simplified.